SPARK-4156 [MLLIB] EM algorithm for GMMs

Implementation of Expectation-Maximization for Gaussian Mixture Models.

This is my maiden contribution to Apache Spark, so I apologize now if I have done anything incorrectly; having said that, this work is my own, and I offer it to the project under the project's open source license.


Use "{" instead of "(" for multi-line foreach/map/etc. calls:
```
breezeData.foreach { x =>
```
Also, no need for "{" to wrap block on right-hand side of "=>"